A bend stiffener serves to locally protect an elongate flexible member from excessive curvature under bending loads. The flexible member in question may for example be a subsea pipe such as a riser used to conduct hydrocarbons from the sea floor to a production platform, but may be any of a wide range of risers, pipelines, flowlines, umbilicals, power cables, tension cables, streamers or the like, according to the application. Bend stiffeners are often—but not always—used underwater.
It must be understood that although the term “flexible” used in relation to the underwater member on which the bend stiffener is to be mounted implies that the member is capable of flexure, the member in question may in practice be a substantial structure with a high degree of stiffness, as in the case of a large oil riser which bends due to the large moments applied to it.
One known form of bend stiffener 10 is represented, in simplified form, in FIG. 1 and comprises a frusto-conical stiffener body 12 with a cylindrical through-going passage (which is internal detail not seen in this drawing) receiving and embracing a flexible member 14 passing though the bend stiffener. A relatively rigid root coupling 16 comprising a flange serves to mount a wider root 18 of the bend stiffener to some fixed structure (not shown). In this way the stiffener body 12 is mounted in cantilever fashion, its root 18 being fixed and its narrower, free, end 20 being able to move as the stiffener body 12 and the member 14 within it flex under a bending load. The drawing shows the stiffener body 12 to be curved but this is the effect of such loading, in the absence of which the stiffener body 12 is straight in this example. The stiffness of the frusto-conical body reduces progressively from the root 18 to the free end 20 and in this way the bend stiffener distributes a bending moment over its length, ensuring that the riser is not subject to a localised—and potentially large—bending moment where it emerges from the fixed structure. The fixed structure in question may for example be an “I” tube on a production platform such as an oil rig. Note that although this is fixed in the sense that it is rigidly anchored to the platform, it is not necessarily static—the platform may be moving according to factors including tide.
A practical example of a bend stiffener having this general form is provided in GB2291686.
Such bend stiffeners are dynamic devices, in that they are subject to and must accommodate variations of load and repeated flexure. They must be designed to protect the flexible member under a range of load cases. They are also required to have a long design lifetime. Fatigue performance must be taken into account to achieve this.
The root coupling 16 needs to be secured to the stiffener body 12 in a durable fashion which enables it to sustain the bending loads applied to the bend stiffener. A known type of root coupling 16 comprises a fabricated steel structure which is incorporated into the stiffener body 12 during its moulding The root coupling typically has features of shape which enable it to engage with the material of the stiffener body 12 and so form a secure and rigid coupling to it. Examples of such couplings are to be found in U.S. Pat. No. 5,526,846 (Maloberti), especially in FIGS. 6, 7 and 8. Structures consisting of welded rods may be used in place of those seen in '846.
The bend stiffener disclosed in GB2291686 is formed as an unbroken cylinder so that mounting it necessarily involves passing it over a free end of the flexible member such as 14. This has some disadvantages. Once the flexible member is installed for use, the end of the flexible member is typically mated to some other structure making removal/replacement of the bend stiffener impossible without disassembly of other parts of the installation.
In the case of a bend stiffener used on a marine riser, for example, a riser end fitting can only be installed once the bend stiffener has been mounted. This means that installing the bend stiffener is a task on the critical path. Delays are potentially expensive. There may be a large lead time in manufacture of a bend stiffener for a particular installation which can lead to users taking risks in project planning. Practical experience shows that this can result in repeated revision of bend stiffener design.
If a bend stiffener of the type found in GB2291686 suffers damage in service, replacing it is a complex process and results in lost production time.
There are known bend stiffeners which are able to be mounted upon the flexible member without access to a free end. FIG. 2 illustrates one such bend stiffener 30 whose stiffener body 32 is split along a line 34, enabling it to be opened out, the stiffener body 32 being resiliently deformed in the process, so that the flexible member is able to be introduced laterally. Integrally moulded upstands 36 on either side of the split 34 receive threaded fasteners 38 at intervals along their length, to close the split line 34.
GB2492109 concerns a bend stiffener whose body is formed in two separable semi-frusto-conical parts for assembly around the flexible member, which are to be held together in use by means of straps passed around their circumference. Loose rings are placed in internal, circumferential troughs in the two body parts to transmit shear from one to the other. This construction provides multiple points of stress concentration considered to limit fatigue lifetime, as well as being somewhat complex in terms of manufacture and assembly. Integrity of the structure depends on maintenance of tension in the straps used to secure it together and creep of the material of the bend stiffener body can lead to loss of this tension. In turn, this results in loss of contact pressure between mating faces of the parts of the bend stiffener body and loss of friction between these faces. It is considered that slip between the faces could become problematic, given the dynamic nature of the loads to which a bend stiffener is exposed, and could eventually lead to an increased risk of structural failure.